The purpose of this study is to describe types and structural properties of instructional activities used by physics and chemistry preservice teachers (N=4) during their teaching of pupils 14 to 16 years old. The study involved the observation and tape recording of 64 classes with all tapes transcribed verbatim. Analysis involved the decomposition of the lesson into segments that have a natural unity of organized action with the objective of finding instructional activities and their structural properties. Activities are categorized by program of action, organizational properties, and structural properties. The dimension of these properties differentiates how the teachers orchestrated the curriculum. Content exposition, laboratory and technical tasks, and problem solving are the activities with the longest duration in these lessons with classroom management being the most frequent instructional activity. Contains 18 references. (Author/DDR)